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Formation of RuO(OH)(2) on RuO2-Based
Electrodes for Hydrogen Production
Lars-Åke Näslund, Arni Sigurdur Ingason, Susanne Holmin and Johanna Rosén
Linköping University Post Print
N.B.: When citing this work, cite the original article.
Original Publication:
Lars-Åke Näslund, Arni Sigurdur Ingason, Susanne Holmin and Johanna Rosén, Formation of
RuO(OH)(2) on RuO2-Based Electrodes for Hydrogen Production, 2014, The Journal of
Physical Chemistry C, (118), 28, 15315-15323.
http://dx.doi.org/10.1021/jp503960q
Copyright: American Chemical Society
http://pubs.acs.org/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-109369
1
Formation of RuO(OH)2 on RuO2-Based Electrodes
for Hydrogen Production
Lars-Åke Näslund,*,† Árni S. Ingason,† Susanne Holmin,‡,§ Johanna Rosen†
†Materials Design, Thin Film Physics Division, Department of Physics, Chemistry, and Biology
(IFM), Linköping University, SE-58183 Linköping, Sweden
‡Permascand AB, SE-840 10 Ljungaverk, Sweden
§Department of Natural Sciences, Mid-Sweden University, SE 870 10 Sundsvall, Sweden
KEYWORDS: Hydrogen evolution, Ruthenium dioxide coatings, XPS, XRD, Phase
transformation, Ruthenium oxyhydroxide
ABSTRACT: The catalytic and durable electrode coating of ruthenium dioxide (RuO2), applied
on nickel (Ni) substrates, is today utilized as electrocatalytic cathodes for hydrogen production,
e.g., in the chlor-alkali process and alkaline water electrolysis. The drawback is, however, the
sensitivity to reverse currents obtained during power shutdowns, e.g. at maintenance, where the
RuO2-based electrodes can be severely damaged unless polarization rectifiers are employed.
Through the material characterization techniques X-ray diffraction and X-ray photoelectron
2
spectroscopy we can now reveal that RuO2 coatings, when exposed to hydrogen evolution at
industrially relevant conditions, transforms into ruthenium oxyhydroxide (RuO(OH)2). The study
further shows that as the hydrogen evolution proceeds the formed RuO(OH)2 reduces to metallic
ruthenium (Ru).
1. INTRODUCTION
The catalytic and durable electrode coatings of ruthenium dioxide (RuO2) or mixtures of RuO2
and other metal oxides, e.g. titanium dioxide (TiO2) and iridium dioxide (IrO2), are widely
employed in a large variety of industrial electrochemical processes, such as chlor-alkali and
chlorate processes, water treatment, electrowinning, and water electrolysis.1,2 As a functionalized
anode, RuO2-based electrodes are often referred to as dimensionally stable anodes (DSA®) and
has been utilized for more than four decades. In addition, these types of electrodes have also
found practical applications as electrocatalytic cathode for hydrogen production, e.g., in the
chlor-alkali process and alkaline water electrolysis,3.4 where the latter represents a promising
candidate for a convenient route for energy storage and conversion.5
However, as a RuO2-based cathode for hydrogen production there is a stability issue
manifested as a coating loss when the cathode is exposed to reverse currents at power shutdowns,
e.g. for maintenance. Several studies have, in addition, reported loss of coating material when
performing polarity inversion tests, i.e. repetitive cycling of the current or potential between the
hydrogen and oxygen evolution reaction or by applying an anodic current after extensive
hydrogen evolution.6-9 The reason for the loss of coating material has, to our knowledge, not
previously been investigated, although it has been suggested that during hydrogen evolution in
alkaline media RuO2 reduces through hydration or hydroxylation processes.8,10-12
3
The aim of the present work is to reveal the chemical process behind the destabilization of the
RuO2 coating when extensively exposed to hydrogen evolution. The material characterization
techniques employed in the study are X-ray diffraction (XRD) for crystal structure information
of the electrode material and X-ray photoelectron spectroscopy (XPS) for chemical environment
information around the probed element. While the bulk probing XRD provides phase-specific
information on the array of atoms in the material, the surface probing XPS provides element-
specific information on the electronic structure of the examined components.
An X-ray diffraction study of RuO2/Ni electrodes for hydrogen production has previously been
performed by Iwakura et al.,8 which showed that the intensity of the RuO2 (110) peak decreased
with time when the cathode was exposed to hydrogen evolution at 3 kA/m2 and 90 °C in 10 M
NaOH electrolyte. In addition the peak in the RuO2 (110) region was shifted toward lower
angles, which the authors speculated being caused by a partial reduction of the RuO2 to lower
valence oxides such as RuOOH. While the former study followed the RuO2 (110) peak progress
for 18 h, Hachiya et al.12 spent 180 days of hydrogen evolution at 6 kA/m2 and 90 °C in 32 w-%
NaOH electrolyte. In the end of the study it was concluded that the RuO2 peaks in the XRD
diffractogram were replaced with peaks that were assigned to hydrated RuO2. In addition there
was a weak feature indicating the presence of metallic Ru. Earlier studies have, thus, shown that
RuO2 transforms into a new phase, although it has not yet been unambiguously identified.
To identify the phase transformation the RuO2/Ni electrodes have previously been investigated
through XPS10,13,14 and the studies suggest a hydroxylation process, mainly because of the
observed intensity redistribution in the O 1s spectrum. As for many transition metal oxides,
interpretation of the Ru 3d spectrum of RuO2 is not a straightforward procedure. Figure 1 shows
the Ru 3d XPS spectra for Ru-metal and for a RuO2/Ni coating which are obtained according to
4
Section 2.3. The Ru 3d XPS spectrum for the Ru-metal shows 3d5/2 and 3d3/2 at 280.2 and 284.3
eV binding energies, respectively, while the RuO2/Ni coating features the two primary spin-orbit
components at 280.7 and 284.9 eV, respectively. Most noticeable is that the full with at half
maximum (fwhm) for the Ru 3d3/2 peak is about twice as large as the Ru 3d5/2 peak, which is
attributed to a Coster-Kronig broadening.15 The larger fwhm is, thus, ascribed to the Ru 3d3/2
photoelectron process that opens up an M4M5N45 Coster-Kronig decay channel, i.e. the electron
core hole in the M shell is filled by an electron from a higher subshell of the same shell while
releasing energy through an Auger electron from the N-shell. The inner shell decay channel will
reduce the lifetime of the Ru 3d3/2 hole state leading to an increased core hole lifetime
broadening of the Ru 3d3/2 peak, i.e. a 3d3/2 peak that has a larger fwhm compare to the 3d5/2
peak. Furthermore, the XPS spectrum for the RuO2/Ni coating shows, in addition to the
dominant low binding energy spin-orbit doublet, two satellites at 282.5 and 286.8 eV,
respectively, which are attributed to final-state screening effects derived from the strong
Coulomb interaction between valence electrons and the core hole produced in the
photoionization process.16,17 Metallic transition metal components can undergo a change in the
core hole screening from mainly extended sp-band screening to mainly localized d-level
screening that results in two different final states, very often denoted unscreened and well-
screened, respectively.18 The photoelectron can leave the atom before or after the change in the
core hole screening and subsequently show main lines and satellites as complex features in the
XPS spectrum, where the more efficient localized d-level screening gives rise to the low binding
energy spin-orbit doublet that in this work is identified as main peaks.
With a confident assignment of all features in the Ru 3d XPS spectra of Ru-metal and RuO2/Ni
coating it has in the present work been possible to identify the chemical process involved when
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RuO2 coatings, exposed to hydrogen evolution at industrially relevant conditions, degenerates
and forms new chemical species sensitive to reverse current.
Figure 1. Ru 3d XPS of (a) the reference RuO2/Ni coating and (b) Ru-metal, fitted with four and
two asymmetric Gaussian-Lorentzian curves, respectively, on Shirley backgrounds. The peak
fitting is performed according to Section 2.4.
2. EXPERIMENTAL SECTION
2.1. RuO2 coating preparation. Most RuO2-based electrodes are produced through thermal
decomposition of ruthenium chloride (RuCl3) salt solutions on a conductive metal substrate
electrode and for alkaline water electrolysis the most common substrate is nickel (Ni), mainly
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because of its high corrosion resistance to highly concentrated alkaline electrolyte at elevated
temperatures. A precursor solution containing 0.60 M Ru was prepared from RuCl3·nH2O salt
from Heraeus (40.27 wt % Ru, analytical reagent grade) and 1-propanol from Fischer Scientific
(analytical reagent grade). Ni disks (Ni-201) with a diameter of 25 mm and thickness of 1 mm
were pre-treated to achieve acceptable adhesion strength of the RuO2 coating to the Ni substrate.
The pre-treatment included blasting with Al2O3 particles (diameter: 50-75 µm), ultrasonically
cleaning in Milli-Q water for 10 min at 30 °C, and rinsing in acetone.
The RuO2 coating was prepared by a conventional multistep precursor solution
application/calcination method. 100 µl of the precursor solution was applied on the Ni disks and
left for 2.5 min. Spin coating was then performed at 4500 rpm for 30 s. Thereafter the sample
was dried in air for 5 min at room temperature and 5 min at 80 °C followed by 12 min
calcinations at 500 °C. This procedure was repeated four times to obtain superimposed layers of
RuO2 with a thickness of 0.4 to 1.4 µm at the thinnest and thickest regions, respectively. On the
fourth application cycle the heat treatment at 500 °C was prolonged to 1 h. Using the described
spin coating application technique, the obtained metal loading became 4.5±0.2 g Ru/m2.
2.2. Electrochemical set-up. The electrochemical cell consisted of three electrodes where the
RuO2-coated Ni disk (cut to a size of 3.1 cm2), a Ni mesh, and a standard calomel electrode (sat.
KCl) from Radiometer Copenhagen were used as a working electrode, a counter electrode, and a
reference electrode, respectively. The RuO2-coated Ni disks and the Ni mesh were attached to
conductors in the form of 25 cm long Ni rods (diam. 1.5 mm) welded to the top end of the
electrodes and mounted in the electrochemical cell at a distance of four cm from each other. The
reference electrode, on the other hand, was operated through a Luggin capillary connected from
an external assembly.
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The electrochemical cell was prepared with a newly made RuO2/Ni electrode, fresh 8 M NaOH
electrolyte and heated to 90.0±0.2 °C prior to each hydrogen evolution run. The electrolyte was
prepared from NaOH pellets obtained from Scharlau (reagent analytical grade) dissolved in
Milli-Q water. To minimize the effect of mass transport limitation on the electrode surface
caused by strong gas evolution, the electrolyte was stirred continuously during each run. The
hydrogen evolution was obtained through galvanostatic polarization at -6.4 kA/m2.
2.3. Material characterization. XRD for phase analysis was performed with an X´pert XRD
system from Panalytical using Ni-filtered Cu Kα radiation. Symmetric -2 scans were operated
using normal Bragg-Brentano geometry. Calibration of the 2 scale was obtained through the
Al2O3 (012)-peak, located at 25.577°. The observed Al2O3 peak is assigned to 50-75 µm particles
incorporated into the surface of the Ni substrates prior to application of the precursor solution.
Chemical bonding states were probed through Ru 3d and O 1s XPS, which were performed
with the AXIS UltraDLD system from Kratos using monochromatic Al Kα radiation. The binding
energy scale of all XPS spectra presented here was calibrated against the Fermi-edge (Ef), which
was set to a binding energy of 0 eV. The overall energy resolution obtained was better than 0.5
eV. Normalization of all spectra was performed at the background on the low binding energy
side of the main peak/peaks.
2.4. XPS spectrum peak fitting. XPS spectrum peak fitting of the Ru 3d XPS spectra for Ru-
metal and for a RuO2/Ni coating are performed using asymmetric Gaussian-Lorentzian (asym-
GL) curves. The peak fitting procedure is constrained using an approach where the two main
peaks, Ru 3d5/2 and Ru 3d3/2, have the same Gaussian but different Lorentzian contributions
caused by the increased core hole lifetime broadening of the Ru 3d3/2 peak.15 For the RuO2/Ni
coating the satellite features are fitted with asym-GL curves where the Lorentzian contribution is
8
the same as obtained for the main peaks Ru 3d5/2 and Ru 3d3/2, respectively, but with twice as
large Gaussian contribution compared to the main peaks. In addition, the two satellite features
need to have the same binding energy shift relative to their main peak. Within these constraints
the asym-GL curves were free to adjust position, height, Gaussian and Lorentzian contributions,
and asymmetry.
Results from XPS spectrum peak fitting of the Ru-metal and the RuO2/Ni coating are included
in Figure 1, which shows that it is enough to use two and four asym-GL curves, respectively, for
a reasonable good fit to the two Ru 3d XPS spectra. Since the samples have been exposed to air
it is plausible that carbon contamination is present on the surfaces19 and therefore show
contributions of C 1s components, i.e. carbon (graphite) at 284.5 eV, hydrocarbon (-CH2- &
CH3) at 285.7 eV, and carboxylate (-COO) at 289.2 eV.20 However, GL curves representing
possible C 1s components were suppressed down to zero intensity when they were included. This
might indicate that the carbon contamination on the Ru-metal and the RuO2/Ni coating sample is
at a very low level or it might suggest that the C 1s features from carbon contamination are
shifted toward higher binding energy compare to what expected and therefore is hidden under the
Ru 3d3/2 component in the spectra. That C 1s contribution might be hidden under the Ru 3d3/2 of
the RuO2 spectrum may explain why the intensity distribution over the Ru 3d region shows a 3d
doublet peak ratio of 3:3.8 instead of 3:2. On the other hand, the 3d doublet peak ratio of the Ru-
metal is 3:2.1, which supports the suggestion of negligible amount of C 1s contamination on the
samples. Alternative explanations for the extra high intensity in the Ru 3d3/2 region of the
RuO2/Ni coating could then be that: the Shirley loss function, which is used for the background
subtraction, does not represent the intrinsic loss structure well enough to completely remove the
background contribution before the peak fitting,21 the Ru 3d3/2 region of the RuO2/Ni coating
9
also consists of intensity from Ru 3d5/2 features, e.g. because of final-state screening effects,18 or
the electrode coating consists of hydrated or hydroxylated (or oxyhydroxylated) Ru-species with
the Ru 3d5/2 component shifted into the Ru 3d3/2 region of RuO2. The present study suggests the
last.
The peak fitting of an XPS spectrum of a RuO2/Ni electrode that has been exposed to hydrogen
evolution was performed with a fitting procedure that included the experimentally obtained
spectra of the reference RuO2/Ni coating and the Ru-metal shown in Figure 1, four spectra of the
RuO2 main peaks obtained from the peak fitting of the reference RuO2/Ni coating, an
experimentally obtained spectrum of C 1s contamination on TiO2, and a Shirley background for
the peak fitted background-free RuO2 main peaks. The experimentally obtained spectra of the
reference RuO2/Ni coating, the Ru-metal, and the C 1s contamination on TiO2 were only free to
adjust their intensity while the RuO2 main peaks (Ru 3d5/2 and Ru 3d3/2) obtained from the peak
fitting of the reference RuO2/Ni coating were free to adjust both their intensity and position on
the binding energy (BE) scale (the intensity ratio and the BE difference between the Ru 3d5/2 and
Ru 3d3/2 peaks were the same as obtained in the peak fitting of the reference RuO2/Ni coating).
No peaks were allowed to change their fwhm. This peak fitting procedure reduced the number of
parameters significantly without loosing meaningful information about the chemical
environment around the probed Ru atoms and at the same time retaining the constrains used in
the peak fitting of the RuO2 reference, i.e. with the same Gaussian and Lorentzian contributions,
peak asymmetries, and the relative binding energy positions and heights of the Ru 3d5/2 and Ru
3d3/2 spin-orbit components intact.
The peak fitting of the O 1s spectra assumed that six asym-GL curves must be included in the
peak fitting procedure: one asym-GL curve that represents RuO2, one asym-GL curve that
10
represents H2O, two asym-GL curves that represent the Ni-O components,22 and two asym-GL
curves that represent the unknown phase. In addition, the peak fitting were constrained in the
way that each asym-GL curve, with the same shape and location, must be present in all fitted O
1s spectra.
Since the spectra originate from samples with a complex mixture of components, e.g. different
chemical components, different core hole lifetime broadening for the different chemical
components as well as for the 3d3/2 and the 3d5/2 spin-orbit components, different final state
screening of the produced core hole, etc., the fit to the spectrum would be better with more peaks
and with variations in peak shapes. However, the drawback would be that the important
information for this study would be harder to extract from the crowd of peaks that would fill the
figures. We have therefore selected to use the minimum number of peaks and peak shapes to
highlight the important information gained from the peak fitting.
3. RESULTS AND DISCUSSION
3.1. X-ray diffraction. Six newly made RuO2/Ni electrodes were, with a current density of -
6.4 kA/m2, exposed to hydrogen evolution in a fresh 8 M NaOH electrolyte at 90.0 °C. The
durations for hydrogen production for each RuO2/Ni electrode were 0.5, 3, 6, 12, 18, and 24 h.
The XRD diffractograms of the RuO2 (110) peak region, presented in Figure 2, show that while
exposed to hydrogen evolution the electrode coating transforms from the rutile phase RuO2 into
another phase. This phase transformation has previously not been identified, to our knowledge,
although it has been observed in earlier studies.8,12
Included in Figure 2 is the RuO2 (110) peak from a reference electrode, i.e. a RuO2/Ni
electrode that has not been exposed to electrolyte. The rutile RuO2 (110) peak of the reference
electrode is located at 27.96°. However, while exposed to hydrogen evolution the electrode
11
coating absorbs hydrogen, which is incorporated into the rutile RuO2 crystal structure.23 The
incorporation of hydrogen, which causes a lattice expansion, is not totally reversible when the
cathodic polarization is removed.23 Hence, after 0.5 h exposure to hydrogen evolution the rutile
RuO2 (110) peak is shifted to 27.90°, i.e. because of the residual hydrogen in the electrode
material there is a 2 shift of 0.06° for the rutile RuO2 (110) peak. As the duration of the
hydrogen evolution proceeds the rutile RuO2 (110) peak shows an increasing shift to lower 2,
accompanied by a decrease in intensity. Simultaneously, a peak from an unknown crystal phase
appears, which increases in intensity with the duration of hydrogen evolution. The inserts in
Figure 2 show Gaussian-Lorentzian (GL) curves obtained from curve fitting of the rutile RuO2
(110) peak in the XRD diffractograms. The peak position, full width at half maximum, and
normalized integrated area of the presented GL curves are summarized in Table 1 and 2 for the
peaks of RuO2 and the unknown phase, respectively. The GL curves for the RuO2 (110) peak
verify a continuous decrease of the integrated intensity and a shift to lower 2. The GL curves
for the peak of the unknown phase show initially an increase of the integrated intensity and
simultaneously a shift to lower 2. However, at extended exposures to hydrogen evolution, i.e.
after 6 h, the integrated intensity suggests no further significant growth of the unknown phase.
12
Figure 2. X-ray diffractograms of RuO2/Ni electrodes exposed to hydrogen evolution. The 2
scale is calibrated against the Al2O3 (012)-peak observed at 25.577° and the intensity is
normalized at the low 2 side of the peaks. The inserts show Gaussian-Lorentzian curves fitted
to the two main features in the diffractogram and represent the peaks of (a) an unknown phase
and (b) rutile RuO2.
The integrated intensity for the GL curves of the peaks of the RuO2 (110) and the unknown
phase are shown in Figure 3. The most prominent observation is that while the peak of the
unknown phase shows no significant increase in size after 6 h of hydrogen evolution, the
decrease of the RuO2 (110) peak continues exponentially. Assuming that the continued
consumption of the crystalline RuO2 after 6 h of hydrogen evolution remains to be because of
13
formation of the unknown crystalline phase, which is reasonable since the exponentially decrease
of the RuO2 (110) peak is not affected when the increase of the peak for the unknown phase
levels out, then the lack of increase of the amount of unknown phase suggest that this too is
consumed by a second process forming a second unknown phase and after 6 h of hydrogen
evolution these two processes will reach a stage when the rate of formation and the rate of the
consumption of the unknown phase are equal. A wide -2 scan does not indicate any formation
of a second unknown phase, which implies that the second unknown phase does not form a
crystal phase or that the amount is not large enough to be detected by the present XRD settings.
Hence, while the consumption of crystalline RuO2 occurs through the whole coating, the
formation of the second unknown phase suggests being limited to the near surface region of the
electrode coating.
Table 1. Parameters Obtained for the Gaussian-Lorentzian Curve Fitting of the RuO2 (110) Peak
duration
[h] 2-value [°] fwhma [°] normalized
integrated area
0 27.96 ±0.01 0.63 ±0.01 1
0.5 27.90 ±0.01 0.62 ±0.01 0.83 ±0.01
3 27.83 ±0.01 0.76 ±0.01 0.58 ±0.01
6 27.68 ±0.01 0.90 ±0.02 0.41 ±0.01
12 27.45 ±0.04 1.09 ±0.07 0.25 ±0.01
18 27.45 ±0.07 1.21 ±0.14 0.21 ±0.02
14
24 27.22 ±0.15 1.32 ±0.31 0.18 ±0.03
afull width at half maximum
Table 2. Parameters Obtained for the Gaussian-Lorentzian Curve Fitting of the Peak of the
Unknown Phase
duration
[h] 2-value
[°]
fwhma
[°]
normalized
integrated area
0 - - -
0.5 27.10 ±0.01 1.41 ±0.26 0.25 ±0.01
3 26.91 ±0.01 1.34 ±0.04 0.71 ±0.07
6 26.69 ±0.01 1.18 ±0.04 0.96 ±0.10
12 26.50 ±0.01 1.00 ±0.05 0.99 ±0.09
18 26.55 ±0.04 0.97 ±0.06 0.97 ±0.07
24 26.48 ±0.03 0.93 ±0.05 1
afull width at half maximum
15
Figure 3. Normalized integrated intensity of the Gaussian-Lorentzian curves that represent the
peaks of (a) the unknown phase and (b) the rutile RuO2.
3.2. X-ray photoelectron spectroscopy. From the XRD study we can conclude that the
electrode coating, exposed to hydrogen evolution in alkaline media, is subjected to a
transformation from the rutile RuO2 phase into two unknown phases that previously have not
been identified. To identify the new phases we have employed the chemical specific and local
geometry sensitive XPS technique.
Figure 4 shows the Ru 3d XPS spectra for the RuO2/Ni electrodes that have been exposed to
hydrogen evolution for 0.5, 3, 6, 12, 18, and 24 h together with the spectra of the reference
RuO2/Ni coating and the Ru-metal. As clearly illustrated in the figure, exposing the RuO2/Ni
electrode to hydrogen evolution promotes changes in the material, manifested as an intensity
redistribution in the Ru 3d XPS spectrum. The redistribution of intensity toward higher binding
energies culminates at a hydrogen evolution duration of 12 h and thereafter the intensity shifts
back toward the lower binding energy region. More specific, the trends that are observed, as the
hydrogen evolution proceeds, are that the Ru 3d5/2 peak becomes broader, the Ru 3d5/2 satellite
16
Figure 4. Ru 3d XPS of RuO2/Ni electrodes exposed to hydrogen evolution for (b) 0.5 h, (c) 3 h,
(d) 6 h, (e) 12 h, (f) 18, and (g) 24 h. Included are also the spectra of (a) the reference RuO2/Ni
coating and (h) the Ru-metal.
17
peak decreases in intensity, the intensity around the Ru 3d3/2 peak increases at first but decreases
after 12 h, the Ru 3d3/2 peak becomes broader, a growth of a peak at 289.2 eV that reduces in
intensity after 12 h, and a growth of a peak at 280.2 eV.
To identify the origin of the intensity redistribution observed in Figure 4, peak fitting is
performed for the Ru 3d XPS spectrum of the RuO2/Ni electrode that has been exposed to
hydrogen evolution for 3 h. The spectrum is peak fitted with the use of the experimentally
obtained Ru 3d spectra of the reference RuO2/Ni electrode and the Ru-metal shown in Figure 1,
the RuO2 main peaks obtained from the peak fitting of the reference RuO2/Ni electrode (see
Figure 1), a spectrum of C 1s contamination on TiO2, and a Shirley background for the peak
fitted RuO2 main peaks. The motivation of including the experimentally recorded spectra of the
RuO2/Ni coating, the Ru-metal, and C 1s contamination on TiO2 in the peak fitting is based on
the fact that the coating still consists of unaltered RuO2, the position of the growing peak at
280.2 eV line up with the position of the Ru 3d5/2 for Ru-metal, and C 1s contamination can be
expected on samples that have been exposed to atmosphere.19 The obtained main peaks from the
peak fitting of the reference RuO2/Ni electrode, shown in Figure 1, are included in the peak
fitting of the RuO2/Ni electrode that has been exposed to hydrogen evolution. RuO2 that has lost
charge shows a significant reduction of the satellite feature and the obtained satellite peaks from
the peak fitting of the reference RuO2/Ni electrode, shown in Figure 1, should therefore not be
included.24
The result of the peak fitting is presented in Figure 5 and shows that the broadening of the Ru
3d5/2 can be fitted with three spectra of RuO2 main peaks that are shifted toward higher binding
energies by 0.5-1.3 eV, which indicates a charge transfer away from the Ru atom. The charge
transfer is most likely a consequence of the incorporated hydrogen that needs to be screened and
18
because of small variations in the position of the hydrogen the interaction with the surrounding
Ru atoms will be affected leading to different degree of shift for the Ru 3d5/2 as well as Ru 3d3/2.
Furthermore, among all the Ru 3d3/2 components that contribute to the intensity in the region of
the Ru 3d3/2 peak, there is one spectrum of RuO2 main peaks that is shifted by 4.3 eV, i.e. the
RuO2 3d5/2 peak is moved into the RuO2 3d3/2 region. The result from the peak fitting is
summarized in Table 3 and suggests that after 3 h of hydrogen evolution 2/3 of the RuO2 in the
probed volume remains unaltered, about 13% of the RuO2 has lost charge, about 8% of the RuO2
has transformed into the unknown phase, about 10% of the RuO2 has been reduced to metallic
Ru and about 2% of the intensity contribution originates from C 1s contamination (in addition to
C 1s contribution that might be hidden in the spectra of RuO2).
Figure 5. Ru 3d XPS of the RuO2/Ni electrode exposed to 3 h of hydrogen evolution fitted with
spectra of the reference RuO2/Ni coating (black) and the Ru-metal (green), as shown in Figure 3,
the RuO2 main peaks obtained from the peak fitting of the reference RuO2/Ni electrode (blue –
19
short BE shift, red – large BE shift), a spectrum of C 1s contamination on TiO2 (black), and a
Shirley background for the peak fitted RuO2 main peaks (blue).
A feature that is not included in the peak fitting of the spectrum in Figure 5 is the high binding
energy feature around 292.9 eV. This peak is probably a satellite feature of the Ru 3d3/2 of the
unknown phase and a corresponding Ru 3d5/2 satellite feature would then contribute to intensity
around 288 eV where the fitted result now shows a deviation from the XPS spectrum. Except
that they most likely have the same origin as the satellite features of RuO2 and, if so, would
contribute to the fraction of the unknown phase, we will not further speculate about the nature of
these satellites.
The peak fitting of the Ru 3d spectrum shown in Figure 5 indicates that the unknown phase
consists of Ru atoms that, compared to RuO2, have lost charge and, thus, suggests an increase of
oxidation state or hydroxylated (or oxyhydroxylated) Ru species, although the former suggestion
can be excluded considering the experimental conditions. The observed shift toward higher
binding energy is contra-intuitive since the electrode is operative in a reducing environment.
Further chemical information is therefore needed and that can be obtained from the O 1s XPS. It
is, however, clear that the unknown phase is an intermediate and the electrode material will in
the end reduce down to metallic Ru as shown by the intensity redistribution toward lower
binding energy (see the intensity increase at 280.2 eV in Figure 4).
20
Table 3. The Result from the Ru 3d Spectrum Peak Fitting of the RuO2/Ni Electrode that has
been Exposed to Hydrogen Evolution for 3 Hours
component peak positionsa
[eV]
3d5/2; 3d3/2
fractionb,
c
assigned to:
Reference RuO2/Ni
coating
280.7; 284.9 0.67 unaltered RuO2 coating.
RuO2, main peaks 1 281.2; 285.4 0.08 For the RuO2, main peaks 1-3,
the assignment is RuO2 involved
in charge transfer from Ru
toward incorporated hydrogen.
RuO2, main peaks 2 281.6; 285.8 0.04
RuO2, main peaks 3 282.0; 286.2 0.01
RuO2, main peaks 4 285.0; 289.2 0.08 the unknown phase.
Ru-metal 280.2; 284.4 0.10 metallic Ru
C 1s contamination 285.7 (C 1s)
289.2 (C 1s)
0.02
-----
hydrocarbon (-CH2- & CH3) at
285.7 eV and carboxylate
(-COO) at 289.2 eV.
aThe peak position is determined within ±0.05 eV.
bThe intensity contribution of the added Shirley background is included in the fractions of the
four RuO2 main peaks.
cThe fraction of the Reference RuO2/Ni coating is determined within ±3%. All other fractions
are determined within ±20%.
21
Figure 6. O 1s XPS of RuO2/Ni electrodes exposed to hydrogen evolution for (b) 0.5 h, (c) 3 h,
(d) 6 h, (e) 12 h, (f) 18, and (g) 24 h. Included are also the spectrum (a) the reference RuO2/Ni
electrode.
The O 1s XPS spectra of the RuO2/Ni electrodes are shown in Figure 6. The O 1s will also go
through an intensity redistribution when exposed to hydrogen evolution. The main intensity
shifts from 529.5 eV toward higher binding energies and after 24 h of hydrogen evolution the
22
dominating contribution is around 531.5 eV. In the view of the shift toward higher binding
energies it is plausible to suggest that the coating material is hydroxylated (or oxyhydroxylated)
and/or hydrated, because O 1s binding energies of Ru-OH and Ru-H2O is revealed to be 530.8
and 532.8 eV, respectively;25 after 24 h of hydrogen evolution most of the RuO2 has been
consumed in the near surface region probed by XPS, as shown in Figure 4(g), and we can
therefore expect a dominant amount of Ru-OH and Ru-H2O components in the corresponding O
1s spectrum, see Figure 6(g).
Peak fitting is performed and presented in Figure 7 for the O 1s XPS spectra of the reference
RuO2/Ni coating and the RuO2/Ni electrode that has been exposed to hydrogen evolution for 24
h. According to the XRD diffractogram, shown in Figure 2, the selected XPS spectrum of the
latter has the highest ratio of the unknown phase compared to RuO2, which makes the peak
fitting easier. However, all oxygen containing species that are present in the near surface region
probed by XPS should be included in the peak fitting of the O 1s spectra. We know that the
reference electrode coating consists of RuO2 and NiO and the electrode coating that has been
exposed to 24 h of hydrogen evolution consists of RuO2 and Ni(OH)2; for the determination of
the Ni-species, see Figure 8. In addition, adsorbed water can hardly be avoided and should
therefore also be included. Finally, based on the obtained results from the Ru 3d spectra, we can
expect hydroxylated or oxyhydroxylated Ru-species. Therefore, based on this, we must include
one asym-GL curve representing RuO2, two asym-GL curves representing the Ni-components, 22
one asym-GL curve representing H2O, and two asym-GL curves representing the unknown
phase, i.e. hydroxylated or oxyhydroxylated Ru-species. The minimum number of asym-GL
curves in the peak fitting of the O 1s spectra must therefore be six. The foremost outcome from
the peak fitting is the remarkable decrease of the asym-GL curve that represents RuO2. Instead
23
the dominating asym-GL curves are the two that represent the unknown phase, where the asym-
GL curve assigned to a Ru-O component and the asym-GL curve assigned to a Ru-OH
component corresponds to 27% and 51%, respectively, of all oxygen containing species in the
near surface region probed by XPS.
Figure 7. O 1s XPS of (a) the reference RuO2/Ni coating and (b) the RuO2/Ni electrode exposed
to hydrogen evolution for 24 h. The two spectra are fitted with six asymmetric Gaussian-
Lorentzian curves, where one curve represents RuO2 (black), one curve represent H2O (blue),
two curves represent the Ni-O components22 (green), and two curves represent the unknown
phase (red). Shirley backgrounds are subtracted from the two spectra.
24
The result from the peak fitting is summarized in Table 4 and the obtained peak fitting suggests
that RuO2 is consumed during hydrogen evolution forming an unknown phase that consists of
Ru-O and Ru-OH bonds with the ratio 1:2.
Table 4. The Result from the O 1s Spectra Peak Fitting of the Reference RuO2/Ni Coating and
the RuO2/Ni Electrode that has been Exposed to Hydrogen Evolution for 24 Hours
componenta peak
positionb [eV]
fractionc,d,
Reference
fractionc,e,
24 h HE
assigned to:
RuO2 529.4 0.66 0.07 unaltered RuO2 coating.
unknown phase, curve 1 530.2 0.12 0.27 Ru-O component
unknown phase, curve 2 531.5 0.12 0.51 Ru-OH component
H2O 532.8 0.03 0.11 adsorbed H2Of
Ni-O, curve 1 529.5 0.04 0.01 Ni-O componentg
Ni-O, curve 2 531.1 0.03 0.03 Ni-OH componentg
aComponents represented by asymmetric Gaussian-Lorentzian curves.
bThe peak position is determined within ±0.05 eV.
cThe fractions of the dominant components are determined within ±3%. All other fractions are
determined within ±20%.
dThe reference RuO2/Ni coating.
eThe RuO2/Ni electrode that has been exposed to hydrogen evolution for 24 h.
fConfirmed through ref.25
gConfirmed through ref.22
25
That Ni-O components must be included in the peak fitting of the O 1s spectra is motivated from
the Ni 2p3/2 XPS spectra shown in Figure 8. The figure clearly shows that the coating of the
reference RuO2 electrode contains NiO, with the main peaks at 854.2 and 855.9 eV, which after
24 h of hydrogen evolution is transformed into Ni(OH)2 with the main peak at 855.6 eV;22,26 the
shoulder around 854.7 eV suggest that some NiO still remains. Included in the spectrum of the
latter is also a metallic Ni peak at 853.0 eV,26 which implies reduction of the Ni(II)-component
during hydrogen evolution and, thus, supporting the decrease of the Ni-O components in the O
1s spectra of Figure 7.
Figure 8. Ni 2p3/2 XPS of (a) the reference RuO2/Ni coating and (b) the RuO2/Ni electrode
exposed to hydrogen evolution for 24 h. The two Ni 2p spectra indicate that NiO, which is
present in the RuO2 coating, transforms into Ni(OH)2 and metallic Ni during hydrogen evolution.
3.3. Identification of the unknown phase. In addition to crystal expansion caused by
hydrogen incorporation into the RuO2 rutile crystal structure,23 the combined XRD and XPS
26
study shows that RuO2 coating, when exposed to hydrogen evolution in alkaline medium, is
subjected to a transformation into a previously not identified phase, which in turn transforms into
metallic Ru. The XRD peak position of the unknown phase is around 26.5°, which coincide with
the (101)-peak position of ruthenium oxychloride, RuOCl2,27 however, RuO2 coatings prepared
from ruthenium nitrosyl nitrate, Ru(NO)(NO3)3, confirmed that the unknown phase is not
RuOCl2.
Peak fitting of the Ru 3d XPS spectrum from a RuO2/Ni electrode that has been exposed to
hydrogen evolution for 3 h shows that the Ru atoms in the unknown phase have lost charge, in
comparison to RuO2, which possibly could be through a hydroxylation process. Furthermore, the
peak fitting of the O 1s spectrum of a RuO2/Ni electrode that has been exposed to hydrogen
evolution for 24 h supports the suggestion of a hydroxylation process and that the hydroxylated
or oxyhydroxylated Ru-species has twice as many Ru-OH components as the number of Ru-O
components. The combined XRD and XPS study, thus, demonstrate that the unknown phase is
ruthenium oxyhydroxide, RuO(OH)2. The study further shows that RuO(OH)2 decomposes as the
hydrogen evolution proceeds and the final compound is metallic Ru, as expected from
thermodynamics and anticipated in, e.g., Pourbaix-diagrams.28,29
The chemical transformation from RuO2 into RuO(OH)2 seems to occur in a hydrogen rich
environment with a considerably low barrier and activated by the charge transfer from the RuO2
toward incorporated hydrogen. It is therefore highly plausible that in practical applications, such
as an electrocatalytic cathode for hydrogen production in the chlor-alkali process and alkaline
water electrolysis, the electrode coating after a run in period consists of mainly RuO(OH)2
through the whole electrode coating. There are, however, no indications for that RuO(OH)2 is
less electrocatalytic active compare to RuO2. Yet, the transformation into RuO(OH)2 can affect
27
other important properties of the electrode coating, e.g. the long-term stability. A recent study
shows, for example, that the transformed electrode coating is very sensitive to reverse currents.30
Further studies are therefore motivated especially when the electrochemical conditions are
industrially relevant, i.e. at highly concentrated electrolyte, high temperature, and high current
density.
4. CONCLUSIONS
Material characterization of RuO2/Ni electrodes that have been exposed to hydrogen evolution
was performed using X-ray diffraction and X-ray photoelectron spectroscopy. The results
support the suggestion that hydrogen incorporation into the RuO2 coating expands the RuO2
rutile crystal structure,23 which is confirmed through a small down shift of the rutile RuO2 (110)
peak. The hydrogen incorporation process induces charge transfer from the RuO2 toward the
incorporated hydrogen. In addition, the positively charged RuO2 coating transforms into
RuO(OH)2, which was revealed through the intensity redistribution from known RuO2 features in
the XRD diffractogram and the Ru 3d and O 1s XPS spectra into features related to the
oxyhydoxylated phase.
Hence, the detailed material characterization study provides evidence for RuO2 consumption
on DSA-type electrodes while exposed to hydrogen evolution in industrially relevant conditions,
such as alkaline media and a high current density. The phase transformation is in form of a
nonelectrochemical hydroxylation, forming RuO(OH)2. As the hydrogen evolution proceeds the
formed RuO(OH)2 reduces to metallic Ru.
28
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: +46 (0)13-28 5733.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The study has partly been accomplished through funding from the European Research Council
under the European Communities Seventh Framework Program (FP7/2007-2013)/ERC Grant
agreement no. [258509], the Swedish Research Council (VR) grant no. 642-2013-8020, and the
KAW Fellowship program. S.H. acknowledges funding from the Swedish Research Council
(VR) grant no. 2007-5059.
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